Gold cathode gas laser discharge tube

ABSTRACT

A COLD CATHODE GAS LASER DISCHARGE TUBE OF IMPROVED DESIGN IS DISCLOSED. A CYLINDRICAL CATHODE IS DISPOSED COAXIALLY ABOUT A CAPILLARY DISCHARGE TUBE. A SUBSTANTIAL LONGITUDINAL OVERLAP OF THE CAPILLARY DISCHARGE TUBE AND THE CATHODE PROVIDES A SHORTER STRUCTURE THAN AVAILABLE IN   THE PRIOR ART, WHILE STILL ALLOWING A LARGE AREA CATHODE EMITTING SURFACE FOR LONG-LIFE OPERATION.

Feb. 15, 1972 w. P. KoLB, JR

GOLD CATHODE GAS LASER DISCHARGE TUBE Original Filed Feb. s, 1968Arran/y United States Patent 'Office Re. 27,282 Reissued Feb. 15, 197227,282 COLD CATHODE GAS LASER DISCHARGEI TUBE William P. Kolb, Jr.,Manhattan Beach, Calif., assigner to Hughes Aircraft Company, CulverCity, Calif.

Original No. 3,495,119, dated Feb. 10, 1970, Ser. No.

703,384, Feb. 6, 1968. Application for reissue June 11, 1970, Ser. No.45,602

Int. Cl. H01j 17/04; H013 3/22 U.S. Cl. 313--217 7 Claims Matterenclosed in heavy brackets appears in the original patent but forms nopart of this reissue specification; matter printed in italics indicatesthe additions made by reissue.

ABSTRACT F THE DISCLOSURE IA cold cathode gas laser discharge tube ofimproved design is disclosed. A cylindrical cathode is disposedcoaxially about a capillary discharge tube. A substantial longitudinaloverlap of the capillary discharge tube and the cathode provides ashorter structure than available in the prior art, while still allowing.a large area cathode emitting surface for long-life operation.

Field of the invention This invention relates to gas laser structuresand more particularly to cold cathode gas discharge apparatus for use insuch structures.

Description of the prior art One of the more commonly used methods forpumping the active medium of gas lasers to the necessary inverted energylevel condition is by means of a cathode-anode electron discharge. Inthe past, both hot cathode and cold cathode structures have been used.For relatively low power laser apparatus the cold cathode arrangement isgenerally preferred.

Understandably, much development effort has been expended in the designof gas laser discharge tubes. Such effort has been largely directed tothe development of improved cathode materials and structural design toachieve efficient long-life operation.

Accordingly, it is an object of the present invention to increase thereliability and lifetime of cold cathode laser discharge tubes.

In the past, the cold cathode laser discharge tubes have taken one oftwo basic structural forms. The first structural form utilizes anelongated discharge tube with the cold cathode, and 'frequently theanode as well, mounted in extension tubes or bulbs off of the main axisof the tube. This arrangement, while satisfactory for many applications,requires considerable space to accommodate the extension tubes in whichthe electrodes are mounted.

The second basic structural form, as exemplified in an article entitledCold Cathodes for Possible Use in 6328 A. Single Mode He-Ne Gas Lasersby U. Hochuli and P'. Haldemann, appearing in The Review of ScientificInstruments, vol. 36, No. 10, October 1965 at p. 1403, utilizes acathode which i's coaxially disposed along a longitudinal extension of acapillary discharge tube. Although this arrangement achieves a smallercross-section, it does so at the expense of a greater length. yIn manyapplications, however, it is desirable or necessary to make thedischarge tube as compact as possible while preserving its long-lifeoperating characteristics.

It is therefore another object of the present invention to provide acold cathode laser discharge tube of decreased longitudinal andcross-sectional dimensions.

Summary of the invention In accordance with the principles of thepresent invention, these objects are accomplished with a cylindricalcathode configuration which, to a `substantial degree, is coextensivewith the coaxially disposed capillary discharge tube. Electrons emittedfrom the surface of the cathode traverse a tfolded path through the boreentrance of the capillary discharge tube to the anode where they arecollected. Areas of very high localized fields which give rise to rapidcathode sputtering and shortened lifetime are avoided in the cathodedesign.

A second embodiment utilizing a single cathode in conjunction with twoanodes and two axially aligned capillary discharge tubes provides alonger effective discharge path but with lower discharge voltages thanrequired by the sinle anode embodiment.

Brief description of the drawings The above-mentioned and other featuresand objects of the present invention will become more apparent byreference to the following description taken in conjunction with theaccompanying drawings, in which:

FIG. 1 is a cross-sectional view of a first embodiment of the presentinvention; and

FIG. 2 is a cross-sectional view of a second embodiment of the presentinvention.

Description of the preferred embodiments Referring more particularly tothe drawings, FIG. l is a cross-sectional view of a preferred embodimentoff the present invention. In FIG. 1 there is shown an outer envelope10. Coaxially disposed and structurally integrated iwith envelope 10 isan elongated capillary discharge tube 11 having an open end region 12communicating with the interior region of envelope 10. A first endmember 13, which is sealed to envelope 10, is provided with an axiallyaligned light transmissive window 14. Window 14 is disposed opposite theend region 12 of disch-arge tube 11 and is preferably oriented at theso-called Brewster angle as shown.

A hollow anode 15, formed of conductive material, is coaxially disposedand bonded to the other end of capillary discharge tube 11, where itjoins the narrowed end of envelope 10. Anode 15 can be fabricated of anysuitable conductive material, such as Kovar, which can be readily fusedor bonded to glass. Anode 15 can be provided with an annular flange 16or other suitable means for facilitating mechanical and electricalconnections. It is apparent that the anode shown in FIG. l is merelyillustrative of one possible anode configuration. Other anodeconfigurations can be readily adapted for use in the present inventionif desired.

A Second end member 17, having a second light transmissive window 1.8,is bonded, fused or otherwise joined to anode 15 opposite the second endof capillary discharge tube 11. Envelope 10, capillary discharge ltube11 and end members 13 and 17 are all fabricated of glass, quartz, orother suitable dielectric material, land together with anode 15, form ahollow gas-tight structure, for containing the gaseous medium.

A cathode electrode 19 of cylindrical cross-section is disposed withinenvelope 10, with its outer surface substantially coinciding with theinside surface of envelope 10. Cathode 19 extends longitudinally fromthe end of envelope 10 near end member 13 toward the opposite end. Thereis thus a substantial longitudinal overlap of cathode 13 and capillarydischarge tube 11. An annular supporting ring 20 is mechanically joinedto cathode 19, thereby lending structural support and spacing forcapillary discharge tube 11 and cathode 19.

Many materials have been suggested for use as cold cathodes in laserdischarge tubes. See, for example, the above-cited article Cold Cathodesfor Possible Use in 6328 A. Single Mode HeNe Gas Lasers. In addition, ithas been found that tantalum, having a thin oxide layer, is alsowell-suited for use in the fabrication of cold cathode 19. Supportingring can be formed of the same material or other suitable conductive ordielectric materials without detracting appreciably from the operationof the present invention.

Conductive pins 21 extend through envelope 10 and are joined to cathode`19 by means of spring-like conductors 22. In addition to providingelectrical coupling, pins 21, with conductors 22, also lend structuralsupport to cathode 19. Although two conductive pins 21 are shown in theembodiment of FIG. 1, it is apparent that 'only one is necessary toprovide electrical contact to cathode 19. In practice, however, two,three, or even more may be desirable for mechanical support.

In operation, the gas through which the electrical discharge is to takeplace is conned Within the structure of FIG. 1, usually at a very lowpressure. To establish the electrical discharge a suitable power supplyof conventional design, not shown, is connected between the cathode 19and anode 1S by means of pins 21 and ange 16, respectively. The powersupply, as is well-known in the art, should be capable of providing arelatively high voltage at a relatively low current. The magnitudes ofthe voltage and current are largely determined by the particular designrequirements of the discharge tube.

Electrons emitted xfrom the inner surface of cathode 19, traverse a paththrough the open end 12 of capillary discharge tube 11 on their way tothe inner surface of anode 15 where they are collected. The emittedelectrons, in traversing this path interact with the gas within thedischarge tube, thereby ionizing a portion of the atoms thereof andcreating the desired discharge and energy level population inversion.When used in a laser oscillator structure, the cold cathode laserdischarge tube of FIG. l can be disposed in an appropriate resonantoptical cavity which is provided with output coupling means forextracting a portion of the output wave energy. In the alternative,windows 14 and 18 can be replaced by reflecting members such as mirrorsfor a unitary laser oscillator structure.

A second embodiment of the present invention, capable of providinggreater output power but also utilizing the overlapping cold cathodeconiiguration, is shown in the cross-sectional view of FIG. 2. Theembodiment of FIG. 2 is a symmetrical extension of the embodiment ofFIG. 1, incorporating a single cathode 40 and two anodes 41 and 42. Thecapillary discharge tubes 43 and 44 are both provided with rst endregions 4S and 46 which communicate with the interior region of thedevice. The second ends of capillary discharge tubes 43 and 44 are, asbefore, sealed to an outer envelope 47 which surrounds cathode 40.

Cathode 40 is mechanically supported at either end by first and secondsupporting rings 48 and 49. Conductive pins 50 extend through the wallof envelope 47 and are conductively connected to cathode 40 by means ofspringlike conductors 51. A second pair of pins 52 and springlikecontuctors S3 are similarly provided at the other end region of cathode40. As mentioned hereinabove, all of the pins 50 and 52 are notessential to the operation of the present invention since adequateelectrical coupling is provided by one pin. However, in order to provideincreased structural support for cathode 40, pins and springlikeconductors can be utilized.

To complete the structure of FIG. 2, end members 54 and 55, eachprovided with optically transmissive windows 56 and 57, respectively,are fused or otherwise joined to the outer end regions of anodes 42 and41. Thus, the entire structure forms a gas-tight container for confiningthe gaseous medium therein.

The operation of the device is similar to that of FIG. 1, except thatboth anodes 41 and 42 are coupled to the source of electrical potential.Appropriate means, such as potentiometers, can be provided in the powersupply circuit to equalize the current to each anode, if desired. Itshould be noted that since the embodiment of FIG. 2 essentially consistsof two parallel connected electron discharge paths the current drainupon the power supply will be substantially twice that of the embodimentof FIG. l. However, since the two capillary discharge tubes 43 and 44are coaxially aligned, the discharge length is effectively doubled.

In all cases it is understood that the above-described embodiments aremerely illustrative of but a small number of the many possible specificembodiments which can represent applications of the present invention.Numerous and varied other arrangements, rincluding other envelope andanode configurations, can be readily devised in accordance with theseprinciples by those skilled in the art without departing from the spiritand scope of the invention.

What is claimed is:

1. A gas laser discharge apparatus comprising, in combination:

at least one elongated capillary discharge tube',

a cylindrical cathode electrode coaxially disposed with respect to saidcapillary discharge tube, the projection of said cathode and saidcapillary discharge tube being coextensive over a substantial portion oftheir respective lengths;

an anode electrode disposed at one end of said capillary discharge tube;

envelope means surrounding said cathode and capillary discharge tube;

couxz'ally aligned end members [having axially aligned opticallytransmissive windows], said end members and said envelope being adaptedto provide a gastight structure Ifor confining a gaseous medium therein;and

conductive means extending through said envelope, said conductive meansbeing conductively connected to said cathode.

2. The gas laser discharge apparatus according to claim 1 wherein saidanode is of cylindrical shape and is coaxially disposed at one end ofsaid capillary discharge tube.

3. The gas laser discharge apparatus according to claim 1 wherein saidcathode is fabricated of tantalum.

4. The gas laser discharge apparatus according to claim 3 wherein theinner surface of said cathode comprises a layer of tantalum oxide.

5. A gas laser discharge apparatus comprising, in combination:

first and second elongated capillary discharge tubes, said capillarydischarge tubes being aligned along a common axis;

a cylindrical cathode electrode coaxially disposed with respect to saidaxis, said cathode electrode extending longitudinally a substantiallydistance along the respective lengths of said capillary discharge tubes,

first and second anode electrodes disposed at opposite ends of saidfirst and second capillary discharge tubes, respectively;

envelope means surrounding said cathode and capillary discharge tubes;

coaxally aligned end members [including axially aligned opticallytransmissive windows], said end members and said envelope being adaptedto provide a gas-tight structure for containing a gaseous mediumtherein; and

conductive means extending through said envelope, said I conductivemeans being conductively connected to said cathode electrode. i 6. Thegas laser discharge apparatus according to claim 5 wherein said cathodeis fabricated of tantalum.

5 7. The gas laser discharge apparatus according to claim FOREIGNPATENTS 6 wherein the inner surface of said cathode comprises a 670777012/1967 Netherlands 313 220 layer of tantalum oxide.

ROY LAKE, Primary Examiner References Cited 5 P. C. DEMEO, AssistantExaminer The following reference, cited by the Examiner, are

gkfltrelctord in the patented le of this patent or the original U'S C1.X R UNITED STATES PATENTS 313-220; 331-945 3,396,301 s/196s Kobayashi etal 33194.5 10

3,486,058 12/19'69 Hernqvist 331--94-5

